molecules

Review

Fluorescence Resonance Energy Transfer Systems inSupramolecular Macrocyclic Chemistry

Xin-Yue Lou, Nan Song and Ying-Wei Yang * ID

International Joint Research Laboratory of Nano-Micro Architecture Chemistry (NMAC), College of Chemistry,Jilin University, 2699 Qianjin Street, Changchun 130012, China; louxy1995@icloud.com (X.-Y.L.);nsong15@mails.jlu.edu.cn (N.S.)* Correspondence: ywyang@jlu.edu.cn; Tel.: +86-431-8516-8468

Received: 31 August 2017; Accepted: 28 September 2017; Published: 29 September 2017

Abstract: The fabrication of smart materials is gradually becoming a research focus innanotechnology and materials science. An important criterion of smart materials is the capacityof stimuli-responsiveness, while another lies in selective recognition. Accordingly, supramolecularhost-guest chemistry has proven a promising support for building intelligent, responsive systems;hence, synthetic macrocyclic hosts, such as calixarenes, cucurbiturils, cyclodextrins, and pillararenes,have been used as ideal building blocks. Meanwhile, manipulating and harnessing light artificiallyis always an intensive attempt for scientists in order to meet the urgent demands of technologicaldevelopments. Fluorescence resonance energy transfer (FRET), known as a well-studied luminescentactivity and also a powerful tool in spectroscopic area, has been investigated from various facets, ofwhich the application range has been broadly expanded. In this review, the innovative collaborationbetween FRET and supramolecular macrocyclic chemistry will be presented and depicted with typicalexamples. Facilitated by the dynamic features of supramolecular macrocyclic motifs, a large varietyof FRET systems have been designed and organized, resulting in promising optical materials withpotential for applications in protein assembly, enzyme assays, diagnosis, drug delivery monitoring,sensing, photosynthesis mimicking and chemical encryption.

Keywords: calixarene; cucurbituril; cyclodextrin; host-guest chemistry; pillararene;supramolecular chemistry

1. Introduction

Supramolecular chemistry, since formally proposed and demonstrated to be of great significancein the realization of molecular recognition and assembly through weak and reversible noncovalentinteractions, has attracted considerable attention in the fields of chemistry and materials science [1].Artificial complexes with diverse properties and various functions formed via noncovalent forces havebeen emerging dramatically in recent years [2–4]. The fact that the Nobel Prize in chemistry in 2016was awarded to Sauvage, Stoddart, and Feringa “for the design and synthesis of molecular machines”even further manifested the increasingly recognized importance attached to this newly-explored realmof chemical science [5].

A major branch of supramolecular research lies in macrocycle-based host-guest chemistry.A host-guest system usually consists of two fundamental components—a macrocyclic host and asuitable guest molecule binding to each other and forming an inclusive complex through weakintermolecular forces, such as hydrophobic forces, electrostatic interactions, hydrogen bonding,Van der Waals forces, etc. Additionally, whether the size of the guest matches the cavity of themacrocycle also plays a crucial role [6–9]. The above-mentioned features endow the matching of ahost-guest pair with rational selectivity. In other words, either entity in a host-guest pair would choose

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and connect to each other selectively and spontaneously, thus realizing molecular recognition [1,7].On the other hand, since the noncovalent interactions are weak forces, the host-guest interactionspossess nice reversibility and responsiveness towards various factors in the ambient environmentsuch as pH, light radiation, redox reactions, competitive factors, chemical signals, and biologicalinterfering, etc. [7–10]. This “stimuli-responsive” property of host-guest systems plays a vital role inthe construction of artificial molecular machines and nanoscaled smart materials [11,12].

As any splendid architectures in the world should require elaborately-manufactured buildingblocks, the fabrication of complicated supramolecular complexes in the nanoscale world can only beachieved when the basic molecular entities are well-designed and synthesized. The characteristics ofthe macrocyclic hosts, such as structural rigidity, functionalizing possibilities, inclusion properties, etc.,perform an essential part in the host-guest activities and their potential applications [7]. Since CharlesPetersen first synthesized crown ethers in 1967 and inspired the later development of host-guestchemistry [7], a large amount of macrocyclic compounds have arisen into scientific scope in thesubsequent five decades, among which four individuals are receiving significant research concentration,with cyclodextrins (CDs) as the natural macrocycles, the other three, artificial, namely, calixarenes,cucurbiturils (CBs), and pillararenes (Figure 1) [13–28]. On account of their mature synthetic protocols,comparatively high yields, and versatile chemical modifications, the host-guest properties of theaforementioned four macrocycles are intensely investigated, resulting in the frequent utilization ofthese supramolecular host molecules [7,29].

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recognition [1,7]. On the other hand, since the noncovalent interactions are weak forces, the host-guest interactions possess nice reversibility and responsiveness towards various factors in the ambient environment such as pH, light radiation, redox reactions, competitive factors, chemical signals, and biological interfering, etc. [7–10]. This “stimuli-responsive” property of host-guest systems plays a vital role in the construction of artificial molecular machines and nanoscaled smart materials [11,12].

As any splendid architectures in the world should require elaborately-manufactured building blocks, the fabrication of complicated supramolecular complexes in the nanoscale world can only be achieved when the basic molecular entities are well-designed and synthesized. The characteristics of the macrocyclic hosts, such as structural rigidity, functionalizing possibilities, inclusion properties, etc., perform an essential part in the host-guest activities and their potential applications [7]. Since Charles Petersen first synthesized crown ethers in 1967 and inspired the later development of host-guest chemistry [7], a large amount of macrocyclic compounds have arisen into scientific scope in the subsequent five decades, among which four individuals are receiving significant research concentration, with cyclodextrins (CDs) as the natural macrocycles, the other three, artificial, namely, calixarenes, cucurbiturils (CBs), and pillararenes (Figure 1) [13–28]. On account of their mature synthetic protocols, comparatively high yields, and versatile chemical modifications, the host-guest properties of the aforementioned four macrocycles are intensely investigated, resulting in the frequent utilization of these supramolecular host molecules [7,29].

Figure 1. Molecular structures of the four types of supramolecular macrocycles: (a) CDs, n = 1–3; (b) calixarenes, n = 1–3; (c) CB[n]s, n = 5–8, 10, 13–15; (d) pillararenes, n = 1–11; and their cartoon depictions.

CDs are a family of toroid-shaped compounds formed by the cyclization of six or more glucose molecules (with prefixes -, β-, -, for six, seven, and eight repeating units). Being hydrophilic in the periphery and relatively hydrophobic in the cavity according to their structures, CDs possess unique selectivity on the guest molecules in the formation of host-guest systems. Exploited more than 120 years ago as a natural product, CDs belong to the first generation of macrocycles [13,14].

Calixarenes, as a kind of phenol-formaldehyde macrocyclic oligomers with a cup-like structure, was first discovered from the Bakelite process. Their unique structural skeleton, easy superficial adjustment, and modifiable host-guest properties earned this type of macrocycle sufficient exploration by chemists to widen the relative range of guest molecules. Following crown ethers, calixarenes are the third generation macrocyclic compounds. Their ease of coordination with transition-metal ions is also a key point for exploiting the applications of calixarenes [15,16].

Figure 1. Molecular structures of the four types of supramolecular macrocycles: (a) CDs, n = 1–3;(b) calixarenes, n = 1–3; (c) CB[n]s, n = 5–8, 10, 13–15; (d) pillararenes, n = 1–11; and theircartoon depictions.

CDs are a family of toroid-shaped compounds formed by the cyclization of six or more glucosemolecules (with prefixes α-, β-, γ-, for six, seven, and eight repeating units). Being hydrophilic inthe periphery and relatively hydrophobic in the cavity according to their structures, CDs possessunique selectivity on the guest molecules in the formation of host-guest systems. Exploited more than120 years ago as a natural product, CDs belong to the first generation of macrocycles [13,14].

Calixarenes, as a kind of phenol-formaldehyde macrocyclic oligomers with a cup-like structure,was first discovered from the Bakelite process. Their unique structural skeleton, easy superficialadjustment, and modifiable host-guest properties earned this type of macrocycle sufficient explorationby chemists to widen the relative range of guest molecules. Following crown ethers, calixarenes are

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the third generation macrocyclic compounds. Their ease of coordination with transition-metal ions isalso a key point for exploiting the applications of calixarenes [15,16].

CB[n]s (n = 5–8, 10, 13–15) are synthetic macrocycles with a barrel shape based on repeatingmonomer elements of glycoluril. With the hydrophobic cavity and polar carbonyl groups on theportals, the CB[n]s family possesses good water-solubility and highly-selective binding interactionswith cationic guest molecules driven by several forces including ion-dipole interactions, hydrogenbonding and hydrophobic forces[17,18].

Pillar[n]arenes (n = 5–15), as a newly-synthesized group of macrocyclic hosts consisting ofhydroquinone units connected by methylene bridges at 2,5-positions, have been under intenseinvestigation due to their unique structures, controllable host-guest properties and functionalizingcapabilities since first reported by Ogoshi et al. Additionally, they have been proved to be goodcandidates as host molecules for diverse electron-deficient guests and hydrophobic molecules due totheir robust electron-rich cavity [19–22].

Comprehensive reviews have been published covering the macrocycles presented above andhave provided sufficient resources for reference, so we will not exhaust the characteristics of thesehost molecules; instead, we will focus on one of their budding applications that have just beenexplored in recent years. Just like the phenomenon of mutualism in nature or the synergisticmechanism of enzymes, the realization of multifunction always entails the cooperation of differententities [23]. For instance, to achieve divergent utilities combined with the properties of host-guestchemistry, synthesized macrocyclic hosts are, in most cases, allied to other functional groups toobtain stimuli-responsive smart materials [8,12,24,25]. Existing examples comprise mesoporous silicananoparticles [24,26] or metal organic frameworks [25] gated by supramolecular switches for drugdelivery. In recent periods, increasing attention is being paid to the combination of host-guest pairsand dye molecules to yield various supramolecular systems with unique luminescent properties,such as fluorescence-enhanced supramolecular polymers [27,28], multicolour photoluminescencehost-guest complexes [30,31], and so on. One boosting approach of fabricating functional fluorescentsupramolecular materials is merging host-guest interactions with fluorescence resonance energytransfer (FRET) effects [32,33] for the purpose of tailoring novel dynamic fluorescent materials withsupramolecular properties.

Known as one of the most powerful spectroscopic technologies, FRET remains a focus of interestamong researchers since it was first proposed theoretically by Theodor Förster, and then highlightedby the discovery of green fluorescent proteins (GFPs) [34,35]. Since detailed physical mechanismsare beyond the covering region of this review, we give a brief explanation of FRET herein. Resultingfrom dipole-dipole coupling, a FRET phenomenon involves at least two fluorophores, known as the“donor” and “acceptor”, respectively [34]. To guarantee the occurrence of energy transfer, a sufficientoverlap between the emission band of one molecule and the absorption band of the other is alwaysrequired, which is fundamental to a FRET process, considering the well-studied nature of molecularelectronic excitation and energy release [33]. Apart from the emission/absorption spectra, anothertwo factors—the donor-acceptor distance and the relative orientation—also play crucial roles in thefluorescent activities of the FRET pairs [36]. Hence, while in proper proximity, as well as orientationto each other, as the donor molecule is excited by light, the energy released by its excited electronsis transferred to the accepter. As a result, either enhanced luminescent emission of the acceptor isperformed visually, or else dark quenching can be expected [37]. Notably, the rate of a FRET effectdepends strongly on the distance between the donor and acceptor. Equation (1), promoted by Förster,expresses this dependence nicely:

E =R6

0

R60 + r6

(1)

where E represents the energy transfer efficiency and r stands for the donor-acceptor distance. R0 isa parameter known as the Förster radius, which varies among different donor-acceptor pairs anddepends highly on their spectral overlap. When the donor-acceptor distance is equal to R0, the transfer

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efficiency equals 50%. It could be inferred that the detectable FRET efficiency would undergo anapparent shift as the donor-acceptor distance changes and, as a result, fluorescent colour variationsare exhibited, which could be observed by naked eyes. Meanwhile, the orientation between the FRETpair also has a large impact on the efficacy of the energy transfer on account of the dipole-dipoleinteractions amongst the entities [34]. Accordingly, due to the high sensitivity to the donor-acceptordistance alteration and visual characteristics, FRET gained the reputation of “molecular spy” or“spectroscopic ruler” [37].

Generally, three types of fluorescent activities in a FRET process are essentially exploited, namely,the turn-off effect on the donor, the switch-on fluorescence of the acceptor and, sometimes, thequenching of both portions in the cases where the acceptor possesses fluorescence quenching ability.The exploitation of these activities in a FRET process requires controllable changes to occur in themutual proximity between the donor and the acceptor [34]. However, several major drawbacks existin the conventional methods of constructing FRET-capable fluorescent materials adopted previously,including the lack of tunability due to the covalent binding manners of the FRET pairs, poor recognizingability between individual molecules (or functional groups) and low compatibility in water phases orbiological systems [33].

Remarkably, the integration of FRET effects with host-guest chemistry, as a promising resolutionto this hindrance, successfully furnished the association and dissociation of the donor-acceptor pairwith extraordinary flexibility and controllability due to the distinct properties of stimuli-responsivenessand the selectivity possessed by host-guest interactions. This breakthrough has astoundingly inspireda variety of applications of FRET, including real-time in vivo monitoring of biomolecules, such as DNAand proteins, and structural manipulation [38–40], cell imaging, drug delivery [41,42], chemical [43,44]and biological sensing [45–47], photosynthesis mimicking [48,49], and other relative scientific areas,which we will explore in the following part.

2. Basic Studies for FRET Systems Based on Host-Guest Chemistry

The simplest and most fundamental way to build up a FRET system with designed host-guest pairsserving as scaffolds or bridges is constructing dye-containing rotaxanes or mechanically-interlockedmolecules (MIMs), in that the earliest studies on the localizing effect induced by host-guest chemistryon the fluorophores required a relatively rigid structure for proper placement. For example, Peiet al. reported a [3]rotaxane complex synthesized via click reactions, constituting two FRET donorgroups on the wheels (crown ether) and an acceptor on the axis [50]. Mimicking the natural process ofphotosynthesis, intramolecular energy transfer was observed from the periphery to the molecular core,attributing to the ideal topology determined by the host-guest pairs. Additionally, they improved thiscomplex by constructing a hyperbranched polyrotaxane with a donor moiety linked to three wheelgroups, forming a 2:3 FRET donor-acceptor mode in the multi-rotaxane architecture [51].

Subsequently, proceeding research work tempted to penetrate deeper in the scope by exploring theexternal impacts on the FRET effect imparted by host-guest interactions, with a broader investigatingrange covering both MIMs and pseudorotaxanes. In 2013, Ogoshi et al. reported the fabricationof a FRET system in a rotaxane complex consisting of a dipyrene functionalized pillar[5]arene (H1)as the wheel and an axle with a perylene stopper (D) [52]. Later in 2016, Bitter and coworkersaccomplished the modulation of FRET with water-soluble pillar[5]arenes (WP5) [53]. As depicted inFigure 2, carboxylatopillar[5]arenes were synthesized to serve as the macrocyclic hosts for two dyemolecules, presented as D and A, respectively, forming FRET dyad through host-guest interactions.The fluorescent activities were influenced by the formation of inclusion complex due to its impact onthe polarity of the guests. The authors, thereafter, synthesized a ditopic guest molecule G by linkingD and A through click reaction. The FRET effect was clearly observed in the solution of G. Uponthe addition of WP5, there was an intense enhancement of the emission of G(A) moiety, while thefluorescence of G(D) was quenched.

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Figure 2. (a) The chemical structures of host-guest complexes WP5⊂D, WP5⊂A and G; and (b) the cartoon illustration of WP5⊂G [53]. Reproduced by permission of [53]. Copyright 2016 Elsevier.

From the studies focusing on the constructing methods for the simple MIMs to intricate supramolecular fluorescent systems, more sophisticated FRET materials controlled by host-guest interactions have been gradually emerging during the past few years in divergent manners, including the fabrication of host-guest complexes, supramolecular polymers, supramolecular nanoparticles and the interposition of inorganic elements. Empirically, in almost every single system involving FRET as a major investigating point, to play tricks with light means either giving out signals or harnessing energy. In cases where sensing, detecting or imaging takes the prior part of the research objective, the success of the materials counts on achieving good distinctness and accuracy of the fluorescent signals. Under this circumstance, apparent variation of the fluorescence is required, which means that the chosen host-guest entities must respond to the stimulus in particular condition so that the efficient functionality could be performed. On the other hand, when the emphasis is placed on the transported energy, in a photosynthesis mimicking system, for example, the goal lies in the high efficacy of the energy transfer process and broadening the adsorption band. This requires the emergence of the host-guest complexes must bring the FRET pairs to the suitable and rigid relative position so as to guarantee the energy transfer efficiency. Apart from the above two types of exploitation of the host-guest components, supramolecular macrocycles, especially CDs, were also introduced into FRET systems to form host-guest pairs, mainly for the purposes of improving the water-solubility and biocompatibility or reducing the toxicity [54,55]. Although these studies comprise both host-guest units and FRET elements, the unique advantages of host-guest chemistry were not demonstrated to the fullest, so we will not include them in the main discussion.

Bearing this in mind, more specific manifestation will be introduced in exact relevance to particular applications in constructing host-guest FRET complexes. In the following part of this review, we’ll provide a comprehensive prospectus on the recent development of FRET materials based on host-guest chemistry. Supreme applications for the novel FRET materials supported by host-guest building blocks will be discussed, including protein assembly [38,39], enzyme assays [56], diagnosis [57], drug delivery monitoring [41,42], sensing [43–47], photosynthesis mimicking [48,49], and chemical encryption materials [58,59] achieved by the collaboration.

3. Applications of FRET Systems Based on Host-Guest Chemistry

3.1. Protein Assembly

A vital biological approach involving FRET in host-guest systems is the controlled assembly of biological macromolecules, such as proteins and DNA. In 2010, Brunsveld and coworkers reported the dimerization of monomeric fluorescent proteins induced by the formation of host-guest complex between cucurbit[8]uril and phenylalanine-glycine-glycine (FGG) tripeptide at a ratio of 1:2 [38]. FGG motifs were immobilized on yellow fluorescent protein (YFP) and cyan fluorescent

Figure 2. (a) The chemical structures of host-guest complexes WP5⊂D, WP5⊂A and G; and (b) thecartoon illustration of WP5⊂G [53]. Reproduced by permission of [53]. Copyright 2016 Elsevier.

From the studies focusing on the constructing methods for the simple MIMs to intricatesupramolecular fluorescent systems, more sophisticated FRET materials controlled by host-guestinteractions have been gradually emerging during the past few years in divergent manners, includingthe fabrication of host-guest complexes, supramolecular polymers, supramolecular nanoparticles andthe interposition of inorganic elements. Empirically, in almost every single system involving FRETas a major investigating point, to play tricks with light means either giving out signals or harnessingenergy. In cases where sensing, detecting or imaging takes the prior part of the research objective,the success of the materials counts on achieving good distinctness and accuracy of the fluorescentsignals. Under this circumstance, apparent variation of the fluorescence is required, which means thatthe chosen host-guest entities must respond to the stimulus in particular condition so that the efficientfunctionality could be performed. On the other hand, when the emphasis is placed on the transportedenergy, in a photosynthesis mimicking system, for example, the goal lies in the high efficacy of theenergy transfer process and broadening the adsorption band. This requires the emergence of thehost-guest complexes must bring the FRET pairs to the suitable and rigid relative position so asto guarantee the energy transfer efficiency. Apart from the above two types of exploitation of thehost-guest components, supramolecular macrocycles, especially CDs, were also introduced into FRETsystems to form host-guest pairs, mainly for the purposes of improving the water-solubility andbiocompatibility or reducing the toxicity [54,55]. Although these studies comprise both host-guestunits and FRET elements, the unique advantages of host-guest chemistry were not demonstrated tothe fullest, so we will not include them in the main discussion.

Bearing this in mind, more specific manifestation will be introduced in exact relevance to particularapplications in constructing host-guest FRET complexes. In the following part of this review, we’llprovide a comprehensive prospectus on the recent development of FRET materials based on host-guestchemistry. Supreme applications for the novel FRET materials supported by host-guest buildingblocks will be discussed, including protein assembly [38,39], enzyme assays [56], diagnosis [57],drug delivery monitoring [41,42], sensing [43–47], photosynthesis mimicking [48,49], and chemicalencryption materials [58,59] achieved by the collaboration.

3. Applications of FRET Systems Based on Host-Guest Chemistry

3.1. Protein Assembly

A vital biological approach involving FRET in host-guest systems is the controlled assembly ofbiological macromolecules, such as proteins and DNA. In 2010, Brunsveld and coworkers reported

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the dimerization of monomeric fluorescent proteins induced by the formation of host-guest complexbetween cucurbit[8]uril and phenylalanine-glycine-glycine (FGG) tripeptide at a ratio of 1:2 [38]. FGGmotifs were immobilized on yellow fluorescent protein (YFP) and cyan fluorescent protein (CFP)genetically on the N-termini, which was occupied by methionine, naturally. The host molecule,cucurbit[8]uril, was connected specifically to two FGG groups as the guests in apparent preference tomethionine in the controlled experiment, indicating the selectiveness of the host-guest recognition.Homo-FRET (decrease of anisotropy) and hetero-FRET were observed, respectively, when homo- andheterodimers formed, thus, FRET was taken advantage of to visualize the process. The supramolecularinducer of fluorescent-protein-dimerization is the first example of introducing host-guest systems in theactivities of proteins, providing a novel insight in manipulating the interactions of biological molecules.

Having demonstrated this new pathway for controlled dimerization of proteins withsupramolecular inducers, they continued the research in 2011 by replacing the tripeptides withmethoxynaphthol (Np) and methylviologen (MV), which were linked to CFP and YFP, respectively,and would form a charge transfer complex in the cavity of cucurbit[8]uril [39]. In this case, onlyheterodimers were yielded in the system, and this specific dimerization was envisaged by FRET.Remarkably, the two guest moieties were artificially attached, yielding host-guest complexes andefficiently eliminating unspecified supramolecular interplay with other parts of proteins.

3.2. Enzyme Assays

Apart from the manipulation of biomolecular motions, it is also necessary that we cover theinvestigation on assaying biological functional complexes, such as enzymes. One of the most favourablemethods for the detection of enzymes is monitoring the products yielded in the enzymatic reactions.

In a most recent study, a new pathway of enzymatic cleavage assay based on fluorescentdye capture through forming host-guest pairs was developed by Smith and coworkers [56].Viral neuraminidase (VNA), as an indicator of influenza, was detected taking advantage of itsenzymatic cleavage reactions. A squaraine (a dye molecule)-derived compound with two blockinggroups at both end was synthesized as the enzymatic substrate while a tetralactam macrocycle withanthracene sidewalls (M) functioned as the supramolecular capture agent. Once the blocking groupswere removed by enzyme cleavage, the fragment left of the substrate was threaded into the macrocyclecavity through hydrogen bonding and hydrophobic stacking, conceptually forming a structure of a[2]pseudrotaxane. Placed at an appropriate proximity, FRET occurred from the anthracene group to thesquaraine core, resulting in strong fluorescent signals. Meanwhile, affinity capture beads were utilizedas a heterogeneous capture assay agent with M functionalized on the surface. Furthermore, variousVNA inhibitor drugs were tested, whose efficiency was examined according to their FRET signals.

3.3. Diagnosis

In 2015, Wang and coworkers reported the supramolecular fluorescent nanoparticles tailored forthe detection of hydrogen peroxide (H2O2) in cancer cells by means of FRET [57], which is depictedin Figure 3a. Fluorescein isothiocyanate (FRET donor) modified β-CD (FITC-β-CD) and rhodamineB (FRET acceptor) modified ferrocene (Fc-RB) were synthesized to produce amphyphilic host-guestpairs linked via the host-guest interaction of β-CD and ferrocene, which spontaneously assembledinto nanoparticles in aqueous solution. The key point of H2O2 detection lay in the mechanismthat the ferrocene unit was hydrophobic in its reduction state and hydrophilic in the oxidationstate. Consequently, degradation of the nanoparticles took place with the presence of H2O2 in highconcentrations like in cancer cell cytoplasm, according to the incompatible inclusion of oxidizedferrocene in the hydrophobic cavity of β-CD. The fluorescence colour was also turned from red togreen along with the separation of the FRET pairs. In comparison, the emission remained green inL929 cells due to the lack of H2O2. The responsiveness toward H2O2 concentration provided a visiblesignal of the existence of concentrated H2O2 and also a powerful evidence for distinguishing normaltissues and cancer cells (Figure 3b).

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Figure 3. (a) Nanoparticles assembled by amphiphilic host-guest pairs in the “FRET-on” state responsive to the ferrocene reduction by ambient H2O2; (b) Confocal fluorescence images of HeLa and L929 cells with (i) L929 cells treated with 5 µm FITC-β-CD/Fc-RB; (ii) L929 cells incubated with 50 µm H2O2 and 5 µm FITC-β-CD/Fc-RB for 4 h and (iii) HeLa cells incubated with 5 µm FITC-β-CD/Fc-RB for 1 h at 37 °C [57]. Reprinted with permission from [57]. Copyright 2015 Wiley.

3.4. Drug Delivery Monitoring

Nanocarriers have been tailored for the delivery and controlled release of drugs and biological materials for years. Yet the cargo loading and release of commonly used carriers are hard to monitor, especially under in vivo circumstances. In 2016, Huskens and coworkers reported supramolecular nanoparticles (SNPs) labelled by FRET pairs for drug delivery and responsive imaging taking advantage of the electrostatic forces between the oppositely-charged carriers and cargos, while the charged SNPs were stabilized by the CD though host-guest interactions [60].

Huang and coworkers reported a [2]rotaxane complex with pillar[5]arene (P5) for mitochondria imaging and drug delivery materials [41]. In this study, concepts of aggregation-induced emission (AIE) and aggregation-caused quenching (ACQ) were both introduced in host-guest controlled FRET study. By anchoring a tetraphenylethylene (TPE) unit, a typical AIE-active luminophore, as a stopper on one end of the axle and pillar[5]arene serving as the wheel, a [2]rotaxane was formed with enhanced fluorescence according to the AIE effect. The anticancer drug, Doxorubicin (DOX), and the ACQ agent as well, was covalently linked to the wheel via imine bridges and, thus, was placed close to TPE, leading to dramatic failing of the fluorescent emission due to the ACQ effect, which occurred upon FRET from TPE to DOX. The “dual-fluorescence quenched” complexes later self-assembled into nanoparticles, which could undergo hydrolysis in endo/lysosomes and the breakage of the imine groups, releasing DOX in the cytoplasm. Due to the extra negative membrane potential of mitochondria, it can be recognized by the [2]rotaxane through electrostatic interactions and be lighted up through the recognition.

With this foundation, they continued to fabricate FRET-capable SNPs for DOX delivery assembled by pillar[5]arene-based amphyphilic supramolecular brush copolymers (SBPs) [42]. TPE and 4,4′-bipyridinium derivative (M) moieties were grafted onto the polymer backbone lternately. As shown in Figure 4a, host-guest interactions formed between the M entities and PEG-Biotin (targeting group) functionalized P5. Hence, SBPs were constructed and subsequently self-assembled into SNPs, which exhibited an AIE effect originating from the aggregation of TPE units in the core of the particles. Once DOX was encapsulated into the SNPs, the emission of the system declined because of the FRET from TPE to DOX and the ACQ of the DOX units, resulting in dual-fluorescence quenching. When the guest M was reduced by the intracellular reductase NAD(P)H in an acidic environment from a bicationic entity to its radical cationic state, the binding between them and P5 were intensely weakened, with the association constant dwindling by two orders of magnitude, leading to the detachment of the host-guest pair and the disassociation of the SNPs. In this way, DOX was released from the confinement of the particles, and the fluorescence recovered as well. The controlled drug releasing properties, the distributions in normal tissues and cancer cell inhibition

Figure 3. (a) Nanoparticles assembled by amphiphilic host-guest pairs in the “FRET-on” stateresponsive to the ferrocene reduction by ambient H2O2; (b) Confocal fluorescence images of HeLa andL929 cells with (i) L929 cells treated with 5 µm FITC-β-CD/Fc-RB; (ii) L929 cells incubated with 50 µmH2O2 and 5 µm FITC-β-CD/Fc-RB for 4 h and (iii) HeLa cells incubated with 5 µm FITC-β-CD/Fc-RBfor 1 h at 37 ◦C [57]. Reprinted with permission from [57]. Copyright 2015 Wiley.

3.4. Drug Delivery Monitoring

Nanocarriers have been tailored for the delivery and controlled release of drugs and biologicalmaterials for years. Yet the cargo loading and release of commonly used carriers are hard to monitor,especially under in vivo circumstances. In 2016, Huskens and coworkers reported supramolecularnanoparticles (SNPs) labelled by FRET pairs for drug delivery and responsive imaging takingadvantage of the electrostatic forces between the oppositely-charged carriers and cargos, while thecharged SNPs were stabilized by the CD though host-guest interactions [60].

Huang and coworkers reported a [2]rotaxane complex with pillar[5]arene (P5) for mitochondriaimaging and drug delivery materials [41]. In this study, concepts of aggregation-induced emission(AIE) and aggregation-caused quenching (ACQ) were both introduced in host-guest controlled FRETstudy. By anchoring a tetraphenylethylene (TPE) unit, a typical AIE-active luminophore, as a stopperon one end of the axle and pillar[5]arene serving as the wheel, a [2]rotaxane was formed with enhancedfluorescence according to the AIE effect. The anticancer drug, Doxorubicin (DOX), and the ACQagent as well, was covalently linked to the wheel via imine bridges and, thus, was placed close toTPE, leading to dramatic failing of the fluorescent emission due to the ACQ effect, which occurredupon FRET from TPE to DOX. The “dual-fluorescence quenched” complexes later self-assembledinto nanoparticles, which could undergo hydrolysis in endo/lysosomes and the breakage of theimine groups, releasing DOX in the cytoplasm. Due to the extra negative membrane potential ofmitochondria, it can be recognized by the [2]rotaxane through electrostatic interactions and be lightedup through the recognition.

With this foundation, they continued to fabricate FRET-capable SNPs for DOX delivery assembledby pillar[5]arene-based amphyphilic supramolecular brush copolymers (SBPs) [42]. TPE and4,4′-bipyridinium derivative (M) moieties were grafted onto the polymer backbone lternately.As shown in Figure 4a, host-guest interactions formed between the M entities and PEG-Biotin (targetinggroup) functionalized P5. Hence, SBPs were constructed and subsequently self-assembled into SNPs,which exhibited an AIE effect originating from the aggregation of TPE units in the core of the particles.Once DOX was encapsulated into the SNPs, the emission of the system declined because of the FRETfrom TPE to DOX and the ACQ of the DOX units, resulting in dual-fluorescence quenching. Whenthe guest M was reduced by the intracellular reductase NAD(P)H in an acidic environment from abicationic entity to its radical cationic state, the binding between them and P5 were intensely weakened,with the association constant dwindling by two orders of magnitude, leading to the detachment of thehost-guest pair and the disassociation of the SNPs. In this way, DOX was released from the confinementof the particles, and the fluorescence recovered as well. The controlled drug releasing properties,

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the distributions in normal tissues and cancer cell inhibition efficiency were investigated as shown inFigure 4b–e, demonstrating the nice encapsulation and cancer targeting capacity (DOX concentrationwas higher in tumour treated with DOX-loaded SNPs than with DOX alone in contrast to that in otherorgans). Uniquely, in this study, the proximity/separation of the FRET pair was not controlled byhost-guest interactions directly, but in a rather circuitous, yet subtle, way by manipulating the assemblyof SBPs and FRET at the same time, performing an elegant cooperation between stimuli-responsivenessof host-guest chemistry and fluorescent signalling of FRET.

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efficiency were investigated as shown in Figure 4b–e, demonstrating the nice encapsulation and cancer targeting capacity (DOX concentration was higher in tumour treated with DOX-loaded SNPs than with DOX alone in contrast to that in other organs). Uniquely, in this study, the proximity/separation of the FRET pair was not controlled by host-guest interactions directly, but in a rather circuitous, yet subtle, way by manipulating the assembly of SBPs and FRET at the same time, performing an elegant cooperation between stimuli-responsiveness of host-guest chemistry and fluorescent signalling of FRET.

Figure 4. (a) Procedure of the formation and disassembly of SNPs with DOX loading and dual fluorescence quenching; (b–e) Graphical demonstration for the in vivo effects of the drug-delivering SNPs: (b) blood circulation time of DOX·HCl and DOX-loaded SNPs analyzed through the plasma concentration of DOX after injection; (c) Distributions among the main organs (tumour, heart, liver, spleen, lung and kidney) of DOX at 12 h post-injection; (d) Tumour growth inhibition curves on the HeLa tumour model treated by phosphate buffered saline(PBS), DOX·HCl and DOX-loaded SNPs respectively; (e) The average weight of the tumours of mice bearing HeLa tumours after the aforementioned three different treatments [42]. Copyright 2016. Reprinted with permission of The Royal Society of Chemistry.

3.5. Sensing

The introduction of host-guest chemistry into FRET systems has also inspired relevant studies aiming at tailoring novel chemosensors and biosensors with enhanced properties by offering either rigid molecular scaffolds or selective binding sites for the sensing functionalities. Therefore, it is necessary for us to introduce FRET effects based on host-guest interactions recruited in the fields of nano-sensing with the latest research achievements exhibited.

Among the massive group of fluorescent chemosensors reported in the last few decades, an important impact is the detection of deleterious matter, such as heavy/transition metal ions that are harmful to the environment and human health [61]. According to previous literature, the utilization of tailored macrocycles, especially calix[n]arenes, has already emerged in chemical sensing scope in several cases benefitting from the desired rigidity, changeable conformations, and various possibilities of functionalization of the asymmetric cyclic structure [15]. For instance, calix[4]crown, with the functionality of double-recognition, was covalently linked to pyrene by the lower rims to form a switchable excimer-based chemosensor for lead ions in the work by Kim and coworkers in 2004 [58]. Later, in 2007 and 2010, another two chemosensors were tailored successively for mercury ions [62,63]. Differently, instead of an excimer, FRET acted as the sensing signals in these two systems. The conformational alterations induced by coordination between ions and the recognition units played a crucial part in the sensing process. With these inspirational predecessors, the combination of FRET and host-guest recognition got employed in this research realm in the last few years and proved an effective pathway for sensing detrimental ions.

In 2009, Yu and coworkers reported a FRET-based approach to detect ferric ions ratiometrically via a water-soluble host-guest complex containing donor (dansyl group)-linked β-CD and acceptor (spirolactam rhodamine)-linked adamantine [43]. A ring-opening reaction was induced by ferric ions efficiently in the acceptor by coordination, reforming the moiety into a longer-wavelength fluorophore comparing to the dansyl part, hence, switching on the FRET. The quantity of Fe3+ could

Figure 4. (a) Procedure of the formation and disassembly of SNPs with DOX loading and dualfluorescence quenching; (b–e) Graphical demonstration for the in vivo effects of the drug-deliveringSNPs: (b) blood circulation time of DOX·HCl and DOX-loaded SNPs analyzed through the plasmaconcentration of DOX after injection; (c) Distributions among the main organs (tumour, heart, liver,spleen, lung and kidney) of DOX at 12 h post-injection; (d) Tumour growth inhibition curves onthe HeLa tumour model treated by phosphate buffered saline(PBS), DOX·HCl and DOX-loadedSNPs respectively; (e) The average weight of the tumours of mice bearing HeLa tumours after theaforementioned three different treatments [42]. Copyright 2016. Reprinted with permission of TheRoyal Society of Chemistry.

3.5. Sensing

The introduction of host-guest chemistry into FRET systems has also inspired relevant studiesaiming at tailoring novel chemosensors and biosensors with enhanced properties by offering eitherrigid molecular scaffolds or selective binding sites for the sensing functionalities. Therefore, it isnecessary for us to introduce FRET effects based on host-guest interactions recruited in the fields ofnano-sensing with the latest research achievements exhibited.

Among the massive group of fluorescent chemosensors reported in the last few decades,an important impact is the detection of deleterious matter, such as heavy/transition metal ionsthat are harmful to the environment and human health [61]. According to previous literature, theutilization of tailored macrocycles, especially calix[n]arenes, has already emerged in chemical sensingscope in several cases benefitting from the desired rigidity, changeable conformations, and variouspossibilities of functionalization of the asymmetric cyclic structure [15]. For instance, calix[4]crown,with the functionality of double-recognition, was covalently linked to pyrene by the lower rims toform a switchable excimer-based chemosensor for lead ions in the work by Kim and coworkers in2004 [58]. Later, in 2007 and 2010, another two chemosensors were tailored successively for mercuryions [62,63]. Differently, instead of an excimer, FRET acted as the sensing signals in these two systems.The conformational alterations induced by coordination between ions and the recognition units playeda crucial part in the sensing process. With these inspirational predecessors, the combination of FRETand host-guest recognition got employed in this research realm in the last few years and proved aneffective pathway for sensing detrimental ions.

In 2009, Yu and coworkers reported a FRET-based approach to detect ferric ions ratiometricallyvia a water-soluble host-guest complex containing donor (dansyl group)-linked β-CD and acceptor(spirolactam rhodamine)-linked adamantine [43]. A ring-opening reaction was induced by ferric

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ions efficiently in the acceptor by coordination, reforming the moiety into a longer-wavelengthfluorophore comparing to the dansyl part, hence, switching on the FRET. The quantity of Fe3+ could becalculated according to the emission spectra. The host-guest complex not only retained the donor andacceptor within their effective distance, but also avoided the unwanted interactions between the twoportions, guaranteeing the structural intactness. Similarly, benefitting from the analogous mechanism,Lü and coworkers developed a ratiometric Hg2+ sensor by grafting FRET-donor-containing conjugatedpolymers on the surface of mesoporous silica nanoparticles (MSNs) as a robust solid scaffold, on whichβ-CDs were anchored and then the acceptor predecessor groups were attached through host-guestrecognition [44]. Fluorescent signals were observed upon the ring-opening effect of Hg2+ on theacceptor precursor. Moreover, this newly-designed sensor was proven capable of detecting Hg2+ inpreference to other metal ions, showing a brilliant advantage in selectivity.

While the sensing of hazardous ions tends to take advantage of coordination-inducedconformational changes to trigger FRET signals, the detection of electroneutral composites, suchas biological compounds, are commonly achieved in an approach of competitive binding. Early in2003, there was already research work reported which employed FRET controlled by competitivehost-guest interactions as a fluorescent probe for conformational studies of helical peptides in aqueoussolution [40]. This stratagem was later used in pharmaceutical or metabolic detections. For example,in 2014, Su and coworkers reported the construction of an amantadine sensor based on competitivehost-guest interactions and a FRET-quenching platform of graphene oxide (GO) [46]. As shown inFigure 5a, β-CDs were immobilized on the graphene oxide sheet covalently as the host moieties. Wellfitting for the inclusion of β-CDs, rhodamine 6G (R6G) served as the fluorescent probe to compete withthe sensing target—amantadine—which could form more stable host-guest pairs with the identicalhosts. Hence, apparent quenching of fluorescence was observed in the system in the absence ofamantadine according to the FRET from R6G to GO, yet once amantadine was introduced, releasingR6G from the GO-CD complex, the fluorescence recovered immediately in a mode linearly relationalto the amount of added amantadine.

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be calculated according to the emission spectra. The host-guest complex not only retained the donor and acceptor within their effective distance, but also avoided the unwanted interactions between the two portions, guaranteeing the structural intactness. Similarly, benefitting from the analogous mechanism, Lü and coworkers developed a ratiometric Hg2+ sensor by grafting FRET-donor-containing conjugated polymers on the surface of mesoporous silica nanoparticles (MSNs) as a robust solid scaffold, on which β-CDs were anchored and then the acceptor predecessor groups were attached through host-guest recognition [44]. Fluorescent signals were observed upon the ring-opening effect of Hg2+ on the acceptor precursor. Moreover, this newly-designed sensor was proven capable of detecting Hg2+ in preference to other metal ions, showing a brilliant advantage in selectivity.

While the sensing of hazardous ions tends to take advantage of coordination-induced conformational changes to trigger FRET signals, the detection of electroneutral composites, such as biological compounds, are commonly achieved in an approach of competitive binding. Early in 2003, there was already research work reported which employed FRET controlled by competitive host-guest interactions as a fluorescent probe for conformational studies of helical peptides in aqueous solution [40]. This stratagem was later used in pharmaceutical or metabolic detections. For example, in 2014, Su and coworkers reported the construction of an amantadine sensor based on competitive host-guest interactions and a FRET-quenching platform of graphene oxide (GO) [46]. As shown in Figure 5a, β-CDs were immobilized on the graphene oxide sheet covalently as the host moieties. Well fitting for the inclusion of β-CDs, rhodamine 6G (R6G) served as the fluorescent probe to compete with the sensing target—amantadine—which could form more stable host-guest pairs with the identical hosts. Hence, apparent quenching of fluorescence was observed in the system in the absence of amantadine according to the FRET from R6G to GO, yet once amantadine was introduced, releasing R6G from the GO-CD complex, the fluorescence recovered immediately in a mode linearly relational to the amount of added amantadine.

Figure 5. Illustrations for (a) the sensing of amantadine by the competitive binding against R6G and (b) the sensing of labetalol by the competitive binding against R6G, accompanied by fluorescence recovery of the freed R6G; (c) Calibration curves of fluorescence intensity of R6G•SCX6-MnO2@RGO proportional to added labetalol concentration [46,47]. Copyright 2014 and 2016. Reprinted with permission of Elsevier and The Royal Society of Chemistry.

Subsequently, Xie et al. published their work on the development of the FRET sensor for labetalol (heart-rate reducer) following the same principle as above. [47] p-Sulfonated calix[6]arene (SCX6) was selected to be the host molecule and Rhodamine 6G (R6G) as the competitive agent (Figure 5b). However, they made a remarkable difference by doping MnO2 on the reduced graphene oxide monosheet to yield MnO2@RGO—a fluorescence quencher with higher efficiency. They further explored on the binding properties of labetalol and R6G with SCX6, coming to a conclusion that labetalol could bind much stronger to SCX6 than R6G, which exactly agreed with

Figure 5. Illustrations for (a) the sensing of amantadine by the competitive binding against R6G and(b) the sensing of labetalol by the competitive binding against R6G, accompanied by fluorescencerecovery of the freed R6G; (c) Calibration curves of fluorescence intensity of R6G•SCX6-MnO2@RGOproportional to added labetalol concentration [46,47]. Copyright 2014 and 2016. Reprinted withpermission of Elsevier and The Royal Society of Chemistry.

Subsequently, Xie et al. published their work on the development of the FRET sensor for labetalol(heart-rate reducer) following the same principle as above. [47] p-Sulfonated calix[6]arene (SCX6)was selected to be the host molecule and Rhodamine 6G (R6G) as the competitive agent (Figure 5b).However, they made a remarkable difference by doping MnO2 on the reduced graphene oxidemonosheet to yield MnO2@RGO—a fluorescence quencher with higher efficiency. They further

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explored on the binding properties of labetalol and R6G with SCX6, coming to a conclusion thatlabetalol could bind much stronger to SCX6 than R6G, which exactly agreed with the FRET signalsof competitive binding. The intensity of the recovered fluorescence was found proportional to theamount of the labetalol added, as depicted in Figure 5c, thus, ratiometric analysis was achieved.

Another significant application of biosensing is the detection and assessment of metabolitesin living organisms. Based upon supramolecular interactions and FRET, Heath and coworkerssuccessfully developed a competitive assay for cellular glutamine (an important metabolite in cancercells) uptake [45]. Cy3-labeled CDs linked to a single-stranded DNA were immobilized throughDNA hybridization on a glass surface modified by the complement sequence, serving as the hostand the FRET donor. Then, a dark quencher molecule—BHQ2—and a glutamine analogue wereconjugated to an adamantane group, the guest, respectively. While the labelled glutamine analoguewas proven capable of being absorbed by cancer cells and selected in a small library of resemblinganalogues. When adamantane-BHQ2 solution was added to the assay surface, FRET from the Cy3group to the BHQ2 was triggered, along with the expected fluorescence quenching. Yet the followingthe introduction of the adamantane-labeled glutamine analogues into the system, on the contrary,retained the fluorescence by way of competing with quenchers over the β-CD hosts since no FREToccurred between Cy3 and the glutamine analogue.

3.6. Photosynthesis Mimicking

As nature provides us an ideal prototype for converting solar energy into chemical energy inthe form of photosynthesis, artificial light harvesting materials have been advancing for a long time,mimicking the antenna structure for the realization of energy transfer. FRET, as an important step in thewhole procedure of photosynthesis, undoubtedly plays an important part in the design and operationof synthesized light harvesting systems. Furthermore, considering the fact that the energy transferringmoieties in nature, namely, the FRET donor and acceptor in the antenna, are commonly held inproximity through noncovalent interactions, it can be expected that the introduction of host-guestchemistry would bring evolving inspiration for organic or hybrid materials to mimic the energytransfer process of photosynthesis.

In the earlier work of MIM-supported FRET systems mentioned in the introduction, the insertionof inclusion effects by designing rotaxane structures effectually promoted the light-harvestingproperties through controlling the donor-acceptor position and suppressing chromophore stacking,making a promising archetype for molecular energy transducting devices [50,51]. However, the fixedconformations of MIMs, though providing the FRET pairs with suitable placement, can be a hindrancefor flexible manipulation of the host-guest light harvesting systems. Instead, the construction of apseudorotaxane would lead to large enhancement in the properties of stimuli-responsiveness throughattachment and detachment between host and guest moieties.

As a further step for more complicated materials mimicking the energy transferring process,Wang, Li, and coworkers fabricated a FRET based supramolecular polymer. Two guest moleculeswith two and three linker entities respectively connected to a boron-dipyrromethene (BODIPY)derivative and a ditopic BODIPY host (H) with two pillarenes on both arms were synthesized. Thus,supramolecular polymers were formed in two binding modes—AA/BB-type and A2B3-type. Boththe polymers exhibited broad absorption bands of UV/visual light and FRET efficiency as highas 51% and 63%, respectively, demonstrating the potential for photosynthesis imitation. With theformation of the supramolecular polymer, host-guest interactions offered proper placement and alsoavoided the unexpected alteration in the donor-acceptor distance, providing a powerful support forthe energy transfer.

3.7. Chemical Encryption

Apart from harvesting solar light and broadening the adsorption spectra of the materials, anothersignificant actualization of manipulating light is the fabrication of photoluminescent materials whose

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fluorescent colours can be well-tuned as a promising tool for the utilizations, such as securityencryption [58] and signal transduction [59].

In 2015, Stoddart and coworkers reported the solid-state fluorescent materials originated froma FRET-capable heterorotaxane (R4•4Cl) with adjustable fluorescent emissions, which is reversiblymanipulated by the supramolecular-controlled aggregation [58] as shown in Figure 6a–b. The basicbuilding motif comprises a FRET pair on the rotaxane axle, bringing about a larger adsorption range,and three macrocyclic wheels threaded on it. The formation of excimers or exciplexes between therotaxanes through π-π interactions led to the aggregation upon increased concentration, the decline ofthe fluorescent emission at 510 nm and the emergence of a broader emission band at 610 nm. Basedon this fact, γ-CD was introduced into the system for the encapsulation of the pyrene groups, and2-adamantylamine hydrochloride (Ad•Cl) as competitive binding agents (CBA). Upon adding γ-CD,the congregation was blocked due to the inclusion effect, destroying the excimers or exciplexes, hence,gradually retrieving the original fluorescence as shown in Figure 6c–d. Yet the addition of Ad•Cl,on the contrary, recovered the aggregation state of the materials, thus, forming a dynamic equilibrium.A wide range of colours of fluorescence from red to green was obtained in the process of tuning theassembly by changing the ratio of R4•4Cl, γ-CD, and Ad•Cl. The solid-state materials were utilised asfluorescent ink with γ-CD as the tonor and Ad•Cl (CBA) as the eraser, providing a large colour assay,which could be coded through a non-linear equation. Interestingly, the hybrid ink exhibited differentcolours when written on different types of paper, possibly resulting from the noncovalent bondingwith the different compositions of the papers (Figure 6c–e). Taking advantage of all these dynamicfluorescent activities controlled precisely by supramolecular equilibria, this novel supramolecularfluorescent material possesses large potential to be applied for encryption coding that could neither betampered nor counterfeited.

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In 2015, Stoddart and coworkers reported the solid-state fluorescent materials originated from a FRET-capable heterorotaxane (R4•4Cl) with adjustable fluorescent emissions, which is reversibly manipulated by the supramolecular-controlled aggregation [58] as shown in Figure 6a–b. The basic building motif comprises a FRET pair on the rotaxane axle, bringing about a larger adsorption range, and three macrocyclic wheels threaded on it. The formation of excimers or exciplexes between the rotaxanes through π-π interactions led to the aggregation upon increased concentration, the decline of the fluorescent emission at 510 nm and the emergence of a broader emission band at 610 nm. Based on this fact, -CD was introduced into the system for the encapsulation of the pyrene groups, and 2-adamantylamine hydrochloride (Ad•Cl) as competitive binding agents (CBA). Upon adding -CD, the congregation was blocked due to the inclusion effect, destroying the excimers or exciplexes, hence, gradually retrieving the original fluorescence as shown in Figure 6c–d. Yet the addition of Ad•Cl, on the contrary, recovered the aggregation state of the materials, thus, forming a dynamic equilibrium. A wide range of colours of fluorescence from red to green was obtained in the process of tuning the assembly by changing the ratio of R4•4Cl, -CD, and Ad•Cl. The solid-state materials were utilised as fluorescent ink with -CD as the tonor and Ad•Cl (CBA) as the eraser, providing a large colour assay, which could be coded through a non-linear equation. Interestingly, the hybrid ink exhibited different colours when written on different types of paper, possibly resulting from the noncovalent bonding with the different compositions of the papers (Figure 6c–e). Taking advantage of all these dynamic fluorescent activities controlled precisely by supramolecular equilibria, this novel supramolecular fluorescent material possesses large potential to be applied for encryption coding that could neither be tampered nor counterfeited.

Figure 6. (a) Chemical structure of FRET-capable [3]heterorotaxane; (b) Schematic illustration of the equilibria involving R4•4Cl as its Cl− salt in the presence of γ-CD and CBAs; (c) Solid-state fluorescence spectra (λex = 347 nm) of R4•4Cl upon adding 0–200 equiv. of γ-CD, followed by 200 equiv. of Ad•Cl as the CBA; (d) Powders obtained from homogeneous mixtures of R4•4Cl and ascending amounts (0–200 equiv) of γ-CD and Ad•Cl (200 equiv) under UV light; (e) Surface-dependent fluorescence ink on different paper media (newsprint, coated and uncoated rag paper, banknotes, copy, matte and glossy white paper) under UV light. Copyright 2015 Nature. Reproduced with permission from [58].

Figure 6. (a) Chemical structure of FRET-capable [3]heterorotaxane; (b) Schematic illustration of theequilibria involving R4•4Cl as its Cl− salt in the presence of γ-CD and CBAs; (c) Solid-state fluorescencespectra (λex = 347 nm) of R4•4Cl upon adding 0–200 equiv. of γ-CD, followed by 200 equiv. of Ad•Clas the CBA; (d) Powders obtained from homogeneous mixtures of R4•4Cl and ascending amounts(0–200 equiv) of γ-CD and Ad•Cl (200 equiv) under UV light; (e) Surface-dependent fluorescenceink on different paper media (newsprint, coated and uncoated rag paper, banknotes, copy, matte andglossy white paper) under UV light. Copyright 2015 Nature. Reproduced with permission from [58].

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Another interesting work lies in the molecular logic gate with FRET as a real-time luminescentresponding mediator. Das and coworkers demonstrated the FRET-facilitated supramolecular assemblywith operational logic functions on solid surface in 2016 [59]. Self-sorting took place by means ofmolecular recognition between the chosen host molecules (crown ether derivatives) immobilized on thesilica surface and different guests as the inputs, with FRET donor and acceptor groups functionalizedon them, respectively. Upon the assembly and disassembly of the host-guest pairs, fluorescent signalswould be produced. Thus, through the self-sorting process, logic operators including YES, INHIBIT,OR, and AND were all achieved with their corresponding luminescent responses.

4. Conclusions and Perspectives

As a conclusion, the recent developments of FRET host-guest systems for responsive materialsand the relative construction strategies have been outlined in this review. The highlighted utilizationof four types of supramolecular host molecules, consisting of the natural product CDs and threeartificial macrocycles, i.e., calixarenes, CBs, and pillararenes, has exhibited great potential in a bunchof applications including protein assembly, enzyme assays, diagnosis, drug delivery monitoring,sensing, photosynthesis mimicking, and chemical encryption, evincing the excellent feasibilityfor supramolecular elements, especially the synthesized macrocyclic molecules, to serve as thefundamental building blocks for the constructions of responsive fluorescent materials.

In the final part of this review, it is necessary to make a summary of the divergent constructingmodes of host-guest based FRET materials for various applications. When the application targets thepharmaceutical regime, especially for in vivo imaging drug delivery, SNPs have been an excellentchoice due to their cargo-encapsulating capacities, fine biocompatibility, and degradability. Withthe FRET agents offering the optical signals for the loading and releasing processes of the cargomolecules, the tailored fluorescent SNPs are able to provide a multifunctional platform for bothcargo delivery and real-time monitoring for the delivering activities instantaneously. However,for diagnostic issues, the constructed systems always contain active functional groups that wouldundergo conformational or electrical changes when triggered by particular external stimuli, leading toeither the assembly/disassembly of the supramolecular complex or the occurrence/expiration of theFRET process, and thus resulting in fluorescent indications for the diagnostic targets. In the sensingpart, intramolecular isomerization and intermolecular competitive binding play a crucial role in theeffectuation of FRET-supported supramolecular sensors by facilitating the responsive FRET signsaccording to the presence or absence of the analytes. Additionally, for biosensing issues, an importantfeature is that the host molecules, as the recognition units, are frequently anchored onto surfaceseither as the receptor of the transferred energy or for solid support. For photosynthesis mimickingmaterials, an ideal approach is fabricating supramolecular polymers with FRET members threadedin the structure. Moreover, taking advantage of the controlled assembly-disassembly events, or thestimuli-responsiveness of host-guest interactions, the mutual distance and orientation of FRET pairsare rendered to be tunable, proving host-guest interactions a powerful tool for altering colours ofthe emitted lights. For another, an obvious benefit is that the polymers can undergo gelation, hence,making desired solid-state smart fluorescent materials. The combination of host-guest interactionswith other optical nanostructures, such as quantum dots, is also a good pathway. As for chemicalencryption, the accuracy and diversity of the fluorescent outputs induced by the switchable dynamicequilibrium is important for efficient secrecy or signal transduction, so a multicomponent system withcontrollable fluorescence variation and a broad range of fluorescent colours is highly required.

In spite of the fact that the innovative cooperation between FRET effects and host-guest chemistryhas inspired the fabrication of a variety of smart fluorescent materials with stimuli-responsiveproperties, there remain many challenges in this upcoming field for researchers to deal with inorder to play better tricks with light. We will list four main aspects requiring further improvementsin the supramolecular FRET-capable materials which have already been constructed up till today:(i) For the photosynthesis mimicking materials, the solidation is comparatively difficult to realize,

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or the solid state would turn out to be fragile and susceptible to diverse influences in the ambientenvironment, owing to the reversible nature of host-guest interactions; (ii) In the biological applications,the multiple factors in vivo should be taken into consideration more excessively. Instead of utilizingartificial stimulating agents, some well-studied reactions in the living organism can be exploiteddirectly as switches, making the responsive FRET systems more particular to different biologicalconditions; (iii) The introduction of nanotechnology into the investigating range would doubtlesslyopen up infinite possibilities for the birth of novel material types and unexpected functionalitiesfor many other optical applications, such as information storage or lab-on-a-chip technology. Thecombination with a large scope of other nanoscaled materials also remains to be realized [64–67];(iv) The design of supramolecular FRET-capable materials would surely benefit from emulatingnatural systems that have been taken advantage of artificially, such as DNA nanomachines. Forexample, a series of synthesized DNA supramolecular nanostructures have already been reported byWillner and coworkers, which include DNA catenane rotary motor, interlocked DNA Olympiadane,DNAzyme-modified MSNs [68–72]. They recruited FRET effects to track the dynamic features ofnanomachines, either for the reconfiguration of the DNA motors or the programmed syntheticprocesses occurring in the pores of DNAzyme gated MSNs. The advances of these DNA nanomaterialswith fluorescence properties might provide inspirations for the constructions of novel supramolecularfluorescent materials based on synthetic macrocycles.

The remarkable development of the synergistically-designed FRET systems supported bysupramolecular macrocyclic chemistry has led to the birth of a variety of smart fluorescent materials.We envision that better tricks with light will be played as bolder innovations should be made,so that more elaborately-designed supramolecular FRET-capable fluorescent systems with tailoredfunctionalities will emerge in the future.

Acknowledgments: We thank the National Natural Science Foundation of China (51673084), the JLU CultivationFund for the National Science Fund for Distinguished Young Scholars, and the Fundamental Research Funds forthe Central Universities for financial support.

Author Contributions: Xin-Yue Lou and Ying-Wei Yang wrote the manuscript. Nan Song provided someillustrations. Ying-Wei Yang directed the project.

Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the designof the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in thedecision to publish the results.

References

1. Lehn, J.-M. Supramolecular chemistry. Science 1993, 260, 1762–1763. [CrossRef] [PubMed]2. Pease, A.R.; Jeppesen, J.O.; Stoddart, J.F.; Luo, Y.; Collier, C.P.; Heath, J.R. Switching devices based on

interlocked molecules. Acc. Chem. Res. 2000, 34, 433–444. [CrossRef]3. Bonifazi, D.; Mohnani, S.; Llanes-Pallas, A. Supramolecular chemistry at interfaces: Molecular recognition

on nanopatterned porous surfaces. Chem.-Eur. J. 2009, 15, 7004–7025. [CrossRef] [PubMed]4. Amabilino, D.B.; Smith, D.K.; Steed, J.W. Supramolecular materials. Chem. Soc. Rev. 2017, 46, 2404–2420.

[CrossRef] [PubMed]5. Nobel prize. Available online: http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2016

(accessed on 28 September 2017).6. Hu, J.; Liu, S. Engineering responsive polymer building blocks with host-guest molecular recognition for

functional applications. Acc. Chem. Res. 2014, 47, 2084–2095. [CrossRef] [PubMed]7. Liu, Z.; Nalluri, S.K.M.; Stoddart, J.F. Surveying macrocyclic chemistry: From flexible crown ethers to rigid

cyclophanes. Chem. Soc. Rev. 2017, 46, 2459–2478. [CrossRef] [PubMed]8. Qu, D.H.; Wang, Q.C.; Zhang, Q.W.; Ma, X.; Tian, H. Photoresponsive host-guest functional systems.

Chem. Rev. 2015, 115, 7543–7588. [CrossRef] [PubMed]9. Yang, Y.W.; Sun, Y.L.; Song, N. Switchable host-guest systems on surfaces. Acc. Chem. Res. 2014, 47,

1950–1960. [CrossRef] [PubMed]

Molecules 2017, 22, 1640 14 of 16

10. Jones, C.D.; Steed, J.W. Gels with sense: Supramolecular materials that respond to heat, light and sound.Chem. Soc. Rev. 2016, 45, 6546–6596. [CrossRef] [PubMed]

11. Cheng, C.; McGonigal, P.R.; Schneebeli, S.T.; Li, H.; Vermeulen, N.A.; Ke, C.; Stoddart, J.F. An artificialmolecular pump. Nat. Nanotechnol. 2015, 10, 547–553. [CrossRef] [PubMed]

12. Klajn, R.; Stoddart, J.F.; Grzybowski, B.A. Nanoparticles functionalised with reversible molecular andsupramolecular switches. Chem. Soc. Rev. 2010, 39, 2203–2237. [CrossRef] [PubMed]

13. Del Valle, E.M.M. Cyclodextrins and their uses: A review. Process Biochem. 2004, 39, 1033–1046. [CrossRef]14. Harada, A. Cyclodextrin-based molecular machines. Acc. Chem. Res. 2001, 34, 456–464. [CrossRef] [PubMed]15. Guo, D.S.; Liu, Y. Calixarene-based supramolecular polymerization in solution. Chem. Soc. Rev. 2012, 41,

5907–5921. [CrossRef] [PubMed]16. Sansone, F.; Baldini, L.; Casnati, A.; Ungaro, R. Calixarenes: From biomimetic receptors to multivalent

ligands for biomolecular recognition. New J. Chem. 2010, 34, 2715. [CrossRef]17. Lee, J.W.; Samal, S.; Selvapalam, N.; Kim, H.-J.; Kim, K. Cucurbituril homologues and derivatives: New

opportunities in supramolecular chemistry. Acc. Chem. Res. 2003, 36, 621–630. [CrossRef] [PubMed]18. Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L. The cucurbit[n]uril family. Angew. Chem. Int. Ed.

2005, 44, 4844–4870. [CrossRef] [PubMed]19. Cragg, P.J.; Sharma, K. Pillar[5]arenes: Fascinating cyclophanes with a bright future. Chem. Soc. Rev. 2012,

41, 597–607. [CrossRef] [PubMed]20. Sathiyajith, C.; Shaikh, R.R.; Han, Q.; Zhang, Y.; Meguellati, K.; Yang, Y.W. Biological and related applications

of pillar[n]arenes. Chem. Commun. 2017, 53, 677–696. [CrossRef] [PubMed]21. Strutt, N.L.; Zhang, H.; Schneebeli, S.T.; Stoddart, J.F. Functionalizing pillar[n]arenes. Acc. Chem. Res. 2014,

47, 2631–2642. [CrossRef] [PubMed]22. Wang, Y.; Ping, G.; Li, C. Efficient complexation between pillar[5]arenes and neutral guests: From host-guest

chemistry to functional materials. Chem. Commun. 2016, 52, 9858–9872. [CrossRef] [PubMed]23. Dong, Z.; Luo, Q.; Liu, J. Artificial enzymes based on supramolecular scaffolds. Chem. Soc. Rev. 2012, 41,

7890–7908. [CrossRef] [PubMed]24. Song, N.; Yang, Y.W. Molecular and supramolecular switches on mesoporous silica nanoparticles.

Chem. Soc. Rev. 2015, 44, 3474–3504. [CrossRef] [PubMed]25. Tan, L.-L.; Li, H.; Qiu, Y.-C.; Chen, D.-X.; Wang, X.; Pan, R.-Y.; Wang, Y.; Zhang, S.X.-A.; Wang, B.; Yang, Y.-W.

Stimuli-responsive metal–organic frameworks gated by pillar[5]arene supramolecular switches. Chem. Sci.2015, 6, 1640–1644. [CrossRef]

26. Li, Z.; Barnes, J.C.; Bosoy, A.; Stoddart, J.F.; Zink, J.I. Mesoporous silica nanoparticles in biomedicalapplications. Chem. Soc. Rev. 2012, 41, 2590–2605. [CrossRef] [PubMed]

27. Song, N.; Chen, D.X.; Qiu, Y.C.; Yang, X.Y.; Xu, B.; Tian, W.; Yang, Y.W. Stimuli-responsive blue fluorescentsupramolecular polymers based on a pillar[5]arene tetramer. Chem. Commun. 2014, 50, 8231–8234. [CrossRef][PubMed]

28. Song, N.; Chen, D.X.; Xia, M.C.; Qiu, X.L.; Ma, K.; Xu, B.; Tian, W.; Yang, Y.W. Supramolecularassembly-induced yellow emission of 9,10-distyrylanthracene bridged bis(pillar[5]arene)s. Chem. Commun.2015, 51, 5526–5529. [CrossRef] [PubMed]

29. Murray, J.; Kim, K.; Ogoshi, T.; Yao, W.; Gibb, B.C. The aqueous supramolecular chemistry of cucurbit[n]urils,pillar[n]arenes and deep-cavity cavitands. Chem. Soc. Rev. 2017, 46, 2479–2496. [CrossRef] [PubMed]

30. Ni, X.L.; Chen, S.; Yang, Y.; Tao, Z. Facile cucurbit[8]uril-based supramolecular approach to fabricate tunableluminescent materials in aqueous solution. J. Am. Chem. Soc. 2016, 138, 6177–6183. [CrossRef] [PubMed]

31. Zhang, Q.W.; Li, D.; Li, X.; White, P.B.; Mecinovic, J.; Ma, X.; Agren, H.; Nolte, R.J.; Tian, H. Multicolourphotoluminescence including white-light emission by a single host-guest complex. J. Am. Chem. Soc. 2016,138, 13541–13550. [CrossRef] [PubMed]

32. Medintz, I.L.; Clapp, A.R.; Mattoussi, H.; Goldman, E.R.; Fisher, B.; Mauro, J.M. Self-assembled nanoscalebiosensors based on quantum dot fret donors. Nat. Mater. 2003, 2, 630–638. [CrossRef] [PubMed]

33. Sapsford, K.E.; Berti, L.; Medintz, I.L. Materials for fluorescence resonance energy transfer analysis: Beyondtraditional donor-acceptor combinations. Angew. Chem. Int. Ed. 2006, 45, 4562–4589. [CrossRef] [PubMed]

34. Scholes, G.D. Long-range resonance energy transfer in molecular systems. Annu. Rev. Phys. Chem. 2003, 54,57–87. [CrossRef] [PubMed]

35. Tsien, R.Y. The green fluorescent protein. Annu. Rev. Biochem. 1998, 67, 509–544. [CrossRef] [PubMed]

Molecules 2017, 22, 1640 15 of 16

36. Hussain, S.A.; Dey, D.; Chakraborty, S.; Saha, J.; Roy, A.D.; Chakraborty, S.; Debnath, P.; Bhattacharjee, D.Fluorescence Resonance Energy Transfer (FRET) sensor. Sci. Lett. J. 2015, 4, 119.

37. Ray, P.C.; Fan, Z.; Crouch, R.A.; Sinha, S.S.; Pramanik, A. Nanoscopic optical rulers beyond the fret distancelimit: Fundamentals and applications. Chem. Soc. Rev. 2014, 43, 6370–6404. [CrossRef] [PubMed]

38. Nguyen, H.D.; Dang, D.T.; van Dongen, J.L.; Brunsveld, L. Protein dimerization induced by supramolecularinteractions with cucurbit[8]uril. Angew. Chem. Int. Ed. 2010, 49, 895–898. [CrossRef]

39. Uhlenheuer, D.A.; Young, J.F.; Nguyen, H.D.; Scheepstra, M.; Brunsveld, L. Cucurbit[8]uril inducedheterodimerization of methylviologen and naphthalene functionalized proteins. Chem. Commun. 2011,47, 6798–6800. [CrossRef]

40. Hossain, M.A.; Mihara, H.; Ueno, A. Novel peptides bearing pyrene and coumarin units with or withoutβ-cyclodextrin in their side chains exhibit intramolecular fluorescence resonance energy transfer. J. Am.Chem. Soc. 2003, 125, 11178–11179. [CrossRef]

41. Yu, G.; Wu, D.; Li, Y.; Zhang, Z.; Shao, L.; Zhou, J.; Hu, Q.; Tang, G.; Huang, F. A pillar[5]arene-based[2]rotaxane lights up mitochondria. Chem. Sci. 2016, 7, 3017–3024. [CrossRef]

42. Yu, G.; Zhao, R.; Wu, D.; Zhang, F.; Shao, L.; Zhou, J.; Yang, J.; Tang, G.; Chen, X.; Huang, F.Pillar[5]arene-based amphiphilic supramolecular brush copolymer: Fabrication, controllable self-assemblyand application in self-imaging targeted drug delivery. Polym. Chem. 2016, 7, 6178–6188. [CrossRef][PubMed]

43. Xu, M.; Wu, S.; Zeng, F.; Yu, C. Cyclodextrin supramolecular complex as a water-soluble ratiometric sensorfor ferric ion sensing. Langmuir 2010, 26, 4529–4534. [CrossRef] [PubMed]

44. Sha, J.; Song, Y.; Liu, B.; Lü, C. Host-guest-recognition-based polymer brush-functionalized mesoporoussilica nanoparticles loaded with conjugated polymers: A facile fret-based ratiometric probe for Hg2+.Micropor. Mesopor. Mat. 2015, 218, 137–143. [CrossRef]

45. Xue, M.; Wei, W.; Su, Y.; Johnson, D.; Heath, J.R. Supramolecular probes for assessing glutamine uptakeenable semi-quantitative metabolic models in single cells. J. Am. Chem. Soc. 2016, 138, 3085–3093. [CrossRef][PubMed]

46. Li, Y.; Gao, Y.; Li, Y.; Liu, S.; Zhang, H.; Su, X. A novel fluorescence probing strategy based onmono-[6-(2-aminoethylamino)-6-deoxy]-β-cyclodextin functionalized graphene oxide for the detectionof amantadine. Sens. Actuat. B-Chem. 2014, 202, 323–329. [CrossRef]

47. Ye, H.; Yang, L.; Zhao, G.; Zhang, Y.; Ran, X.; Wu, S.; Zou, S.; Xie, X.; Zhao, H.; Li, C.-P. A fret-basedfluorescent approach for labetalol sensing using calix[6]arene functionalized MnO2@graphene as a receptor.RSC Adv. 2016, 6, 79350–79360. [CrossRef]

48. Villafiorita-Monteoleone, F.; Daita, V.; Quarti, C.; Perdicchia, D.; Del Buttero, P.; Scavia, G.; Zoppo, M.D.;Botta, C. Light harvesting of cdse/cds quantum dots coated with β-cyclodextrin based host–guest speciesthrough resonant energy transfer from the guests. RSC Adv. 2014, 4, 28886–28892. [CrossRef]

49. Meng, L.B.; Li, D.; Xiong, S.; Hu, X.Y.; Wang, L.; Li, G. Fret-capable supramolecular polymers based ona bodipy-bridged pillar[5]arene dimer with bodipy guests for mimicking the light-harvesting system ofnatural photosynthesis. Chem. Commun. 2015, 51, 4643–4646. [CrossRef] [PubMed]

50. Wang, J.Y.; Han, J.M.; Yan, J.; Ma, Y.; Pei, J. A mechanically interlocked [3]rotaxane as a light-harvestingantenna: Synthesis, characterization, and intramolecular energy transfer. Chem. Eur. J. 2009, 15, 3585–3594.[CrossRef] [PubMed]

51. Han, J.-M.; Wang, X.-Y.; Zhang, Y.-H.; Liu, C.; Pei, J. Main-chain hyperbranched polyrotaxane: Synthesis,photophysical properties, and energy funnel. Polymer 2012, 53, 3704–3711. [CrossRef]

52. Ogoshi, T.; Yamafuji, D.; Yamagishi, T.A.; Brouwer, A.M. Forster resonance energy transfer by formation of amechanically interlocked [2]rotaxane. Chem. Commun. 2013, 49, 5468–5470. [CrossRef] [PubMed]

53. Bojtár, M.; Szakács, Z.; Hessz, D.; Bazsó, F.L.; Kállay, M.; Kubinyi, M.; Bitter, I. Supramolecular fret modulationby pseudorotaxane formation of a ditopic stilbazolium dye and carboxylato-pillar[5]arene. Dyes Pigments2016, 133, 415–423. [CrossRef]

54. Liu, G.; Xu, X.; Chen, Y.; Wu, X.; Wu, H.; Liu, Y. A highly efficient supramolecular photoswitch for singletoxygen generation in water. Chem. Commun. 2016, 52, 7966–7969. [CrossRef] [PubMed]

55. Wang, A.; Jin, W.; Chen, E.; Zhou, J.; Zhou, L.; Wei, S. Drug delivery function of carboxymethyl-β-cyclodextrinmodified upconversion nanoparticles for adamantine phthalocyanine and their nir-triggered cancertreatment. Dalton Trans. 2016, 45, 3853–3862. [CrossRef] [PubMed]

Molecules 2017, 22, 1640 16 of 16

56. Liu, W.; Gomez-Duran, C.F.A.; Smith, B.D. Fluorescent neuraminidase assay based on supramolecular dyecapture after enzymatic cleavage. J. Am. Chem. Soc. 2017, 139, 6390–6395. [CrossRef] [PubMed]

57. Wei, X.; Dong, R.; Wang, D.; Zhao, T.; Gao, Y.; Duffy, P.; Zhu, X.; Wang, W. Supramolecular fluorescentnanoparticles constructed via multiple non-covalent interactions for the detection of hydrogen peroxide incancer cells. Chem.-Eur. J. 2015, 21, 11427–11434. [CrossRef] [PubMed]

58. Hou, X.; Ke, C.; Bruns, C.J.; McGonigal, P.R.; Pettman, R.B.; Stoddart, J.F. Tunable solid-state fluorescentmaterials for supramolecular encryption. Nat. Commun. 2015, 6, 6884. [CrossRef] [PubMed]

59. Gangopadhyay, M.; Maity, A.; Dey, A.; Das, A. [2]Pseudorotaxane formation with fret based luminescenceresponse: Demonstration of boolean operations through self-sorting on solid surface. J. Org. Chem. 2016, 81,8977–8987. [CrossRef] [PubMed]

60. Grana-Suarez, L.; Verboom, W.; Huskens, J. Fluorescent supramolecular nanoparticles signal the loading ofelectrostatically charged cargo. Chem. Commun. 2016, 52, 2597–2600. [CrossRef] [PubMed]

61. Zhang, R.; Yan, F.; Huang, Y.; Kong, D.; Ye, Q.; Xu, J.; Chen, L. Rhodamine-based ratiometric fluorescentprobes based on excitation energy transfer mechanisms: Construction and applications in ratiometric sensing.RSC Adv. 2016, 6, 50732–50760. [CrossRef]

62. Lee, Y.H.; Lee, M.H.; Zhang, J.F.; Kim, J.S. Pyrene excimer-based calix[4]arene FRET chemosensor formercury(II). J. Org. Chem. 2010, 75, 7159–7165. [CrossRef] [PubMed]

63. Othman, A.B.; Lee, J.W.; Wu, J.-S.; Kim, J.S.; Abidi, R.; Thuery, P.; Strub, J.M.; Van Dorsselaer, A.; Vicens, J.Calix[4]arene-based, Hg2+-induced intramolecular fluorescence resonance energy transfer chemosensor.J. Org. Chem. 2007, 72, 7634–7640. [CrossRef] [PubMed]

64. Wang, L.; Yan, R.; Huo, Z.; Wang, L.; Zeng, J.; Bao, J.; Wang, X.; Peng, Q.; Li, Y. Fluorescence resonantenergy transfer biosensor based on upconversion-luminescent nanoparticles. Angew. Chem. Int. Ed. 2005, 44,6054–6057. [CrossRef] [PubMed]

65. Shulov, I.; Rodik, R.V.; Arntz, Y.; Reisch, A.; Kalchenko, V.I.; Klymchenko, A.S. Protein-sized brightfluorogenic nanoparticles based on cross-linked calixarene micelles with cyanine corona. Angew. Chem.Int. Ed. 2016, 55, 15884–15888. [CrossRef] [PubMed]

66. Kim, K.-S.; Kim, J.-H.; Kim, H.; Laquai, F.; Arifin, E.; Lee, J.-K.; Yoo, S.; Sohn, B.-H. Switching off FRET inthe hybrid assemblies of diblock copolymer micelles, quantum dots, and dyes by plasmonic nanoparticles.ACS Nano 2012, 6, 5051–5059. [CrossRef] [PubMed]

67. Gatti, T.; Brambilla, L.; Tommasini, M.; Villafiorita-Monteleone, F.; Botta, C.; Sarritzu, V.; Mura, A.;Bongiovanni, G.; Zoppo, M.D. Near IR to red up-conversion in tetracene/pentacene host/guest cocrystalsenhanced by energy transfer from host to guest. J. Phys. Chem. C 2015, 119, 17495–17501. [CrossRef]

68. Balogh, D.; Garcia, M.A.A.; Albada, H.B.; Willner, I. Programmed synthesis by stimuli-responsivednazyme-modified mesoporous SiO2 nanoparticles. Angew. Chem. Int. Ed. 2015, 54, 11652–11656. [CrossRef][PubMed]

69. Lu, C.H.; Cecconello, A.; Elbaz, J.; Credi, A.; Willner, I. A three-station DNA catenane rotary motor withcontrolled directionality. Nano Lett. 2013, 13, 2303–2308. [CrossRef] [PubMed]

70. Lu, C.H.; Cecconello, A.; Qi, X.J.; Wu, N.; Jester, S.S.; Famulok, M.; Matthies, M.; Schmidt, T.L.; Willner, I.Switchable reconfiguration of a seven-ring interlocked DNA catenane nanostructure. Nano Lett. 2015, 15,7133–7137. [CrossRef] [PubMed]

71. Lu, C.H.; Qi, X.J.; Cecconello, A.; Jester, S.S.; Famulok, M.; Willner, I. Switchable reconfiguration of aninterlocked DNA olympiadane nanostructure. Angew. Chem. Int. Ed. 2014, 53, 7499–7503. [CrossRef][PubMed]

72. Lu, C.H.; Willner, I. Stimuli-responsive DNA-functionalized nano-/microcontainers for switchable andcontrolled release. Angew. Chem. Int. Ed. 2015, 54, 12212–12235. [CrossRef] [PubMed]

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